The contribution of hybrids towards enhancing productivity of crop plants through the phenomenon of heterosis or hybrid vigor, in which F1 hybrid plants generated by crosses between two genetically diverse parents exhibit improved yield than either of the parents, is well documented (Banga 1992, In Breeding Oilseed Brassicas Eds. Labana K S, Banga S S and Banga S K, Narosa Publishing House, New Delhi; Pradhan et. al., 1993, Euphytica 69:219-229). Cross pollination is essential for the production of hybrids and hence, it is imperative to render one of the parents male sterile to ensure cross pollination to facilitate the production of hybrid seeds in a commercially viable manner.
One of the ways to induce male sterility is by the use of cytoplasmic male sterility (CMS) systems. Cytoplasmic male sterility (CMS) in plants is a maternally inherited trait, the genetic determinants of which are located in genomes of the cytoplasmic organelle, the mitochondria, and is manifested as a result of incompatibility between the nuclear and mitochondrial genomes. This incompatibility arises mainly due to aberrant recombination events, insertions and deletions in mitochondrial DNA that result in unusual open reading frames (Dewey et. al., 1987, Proc. Natl. Acad. Sci. USA 84:5374-5378; Krishnasamy and Makaroff, 1993, Curr. Genet. 24:156-163). These rearrangements of mitochondrial DNA molecules are widely reported by Bonhomme et. al. (1992, Mol. Gen. Genet. 235:340-348), Janska et. al. (1998, Plant Cell 10:1163-1180) and have also been shown to occur spontaneously in vitro (Vitrat et. al., 1992, Mol. Gen. Genet. 233:193-200; Kanazawa et. al., 1994, Genetics 138:865-870). In most CMS systems, the mitochondrial DNA rearrangements mainly affect the development of male reproductive organs but may also induce some other floral and phenotypic abnormalities. Such plants are severely impaired in their ability to produce functional pollen grains.
CMS provides a useful mechanism for pollination control and commercial hybrid seed production. In crop plants, where seeds are the product of economic value, restoration of fertility in the F1 hybrids is essential. CMS-restorer systems have been used for the production of hybrid seeds in a large number of crops like maize, rice, sorghum, sunflower etc.
CMS may be either alloplasmic or spontaneous in origin. Spontaneous CMS systems arise in breeding lines without intentional intervention. Examples include: the maize T-cytoplasm [Duvick D N: Cytoplasmic pollen sterility in corn. In: Caspari E W, Thoday J M (eds) Advances in Genetics. Vol 13, pp 1-56. Academic Press, New York (1965)]; the pol cytoplasm of B. napus (which arose in cultivar Polima, Fu, 1981, Eucarpia Cruciferae Newsl. 6:6-7) and the male sterile cytoplasm of Phaseolus (first reported by Basset and Shuh, 1982, J. Am. Soc. Hort. Sci. 107:791-793). The ‘Pol’ CMS has been extensively used to produce CMS lines in oilseed B. napus (Barsby et. al., 1987, Plant Sci. 53:243-248; Fu et. al., 1990, Plant Breeding 104:115-120; Sodhi et. al., 1993, Plant Breeding 110:334-337).
In an alloplasmic CMS system, an alien cytoplasm present in the nuclear background of a cultivated crop variety leads to nuclear—cytoplasmic incompatibility inducing male sterility. Several alloplasmic CMS systems reported in various Brassica species have been generated using wild relatives as the cytoplasmic donors. B. oxyrrhina cytoplasm has been shown to induce male sterility in B. juncea (Prakash and Chopra, 1990, Theor. Appl. Genet. 79: 285-287) and B. napus (Arumugam et. al., 2000, Theor. Appl. Genet. 100:1043-1049). Similarly, B. tournefortii cytoplasm in B. juncea and B. napus (Pradhan et. al., 1991, Plant Breeding 106:204-208; Steiwe and Robellen, 1994, Plant Breeding 113: 294-304; Arumugam et. al., 1996, Theor. Appl. Genet. 92:762-768), Diplotaxis siifolia cytoplasm in B. juncea (Rao et. al., 1994, Plant Breeding 112:171-174), Trachystoma ballii cytoplasm in B. juncea (Kirti et. al., 1995a, Plant Breeding 114:434-438), Raphanus sativus cytoplasm in B. juncea and B. napus (Kirti et. al., 1995b, Theor. Appl. Genet. 91:517-521; Pelletier et. al., 1983, Mol. Gen. Genet. 191: 244-250; Menczel et. al., 1987, Plant Cell Rep. 6:98-101), Moricandia arvensis cytoplasm in B. juncea (Prakash et. al., 1998, Theor. Appl. Genet. 97:488-492) and Erucastrum canariense cytoplasm in B. juncea (Prakash et. al., 2001, Plant Breeding 120:479-482) induces male sterility. A number of alloplasmic CMS systems have also been reported in B. oleracea (Hu et. al., 1997, J. Agric. Sci. 128:299-301, Verma et. al., 2000, Plant Breeding 119:90-92, U.S. Pat. No. 6,046,383, UK Patent No. GB2281568A, WO 96/21010).
However, most of the alloplasmic CMS systems cited above could not be utilized effectively due to lack of corresponding fertility restorer lines that could restore male fertility in the respective F1 hybrids. For such CMS systems, attempts have been made to introgress restorer genes from the respective donors of the cytoplasmic male sterility trait. However, the transfer of restorer genes from wild relatives is often hampered by its linkage to undesirable traits viz. reduced female fertility as observed in ‘Ogu’ CMS in B. napus (Delourme and Renard, 1988, Genome 30:234-238). This undesirable linkage was subsequently broken in order to develop restorer lines with good female fertility (Delourme et. al., 1991, Proc. 8th Int. Rapeseed Congr. 5:1506-1510, Patent No. CA2273137).
Similar attempts have been made to introduce fertility restorer function from the cytoplasm donor species for various alloplasmic CMS systems available in B. juncea. However, this kind of transfer requires long-winded breeding programs and could falter due to lack of chromosomal exchanges or linkage drag. Kirti et. al. (1997, Plant Breeding 116:259-262) reported the isolation of a restorer plant for Trachystoma ballii CMS with 90% pollen viability. However, the identified restorer plant continued to show leaf serration of T. balli, intermediate flower morphology, contorted pods and yellow cylindrical seeds typical of T. balli, indicating that restoration of Trachystoma ballii CMS is far from being perfect.
Prakash et. al. (1998, Theor. Appl. Genet. 97:488-492) reported isolation of a restorer of another CMS system derived from Moricandia arvensis wherein the restored plant showed 96% pollen viability. However, the restored plants exhibited severe chlorosis similar to CMS plants as well as reduced female fertility. Another alloplasmic CMS system in B. juncea derived from Erucastrum canariense cytoplasm developed by Prakash et. al. (2001, Plant Breeding 120:479-482), also lacked proper restoration. The restored plant showed 90% pollen viability but was also associated with reduced female fertility. Recently Bhat et. al. (2005, Plant Breeding 124:117-120) have shown introgression of restorer from Moricandia arvensis into B. juncea which restores both M. arvensis and Diplotaxis catholica cytoplasm.
So far none of the restorer lines isolated for respective alloplasmic CMS systems has shown perfect fertility restoration without any negative effect in terms of morphology, fertility etc. thereby compromising the eventual usage of such lines for the production of hybrid plants and seeds with enhanced yield.
Thus, a need in the art currently exists for developing a cytoplasmic male sterility system along with its restorer for Brassica species which is substantially free of phenotypic infirmities and other defects, and can be fully restored and therefore used in producing hybrid seeds in a commercially viable manner. The present invention seeks to fulfill this need.